One of the earliest of the neoceratopsian dinosaurs, Protoceratops had a well developed frill that extended back from the face and over the neck. However, it lacked the horns of the more advanced members of the group, although some species featured a small bump on the snout that may have supported a keratinous horn, similar to that of a modern rhinoceros.

Protoceratops was a very common animal in the late Cretaceous lowland habitats of Mongolia. Fossilized remains of this dinosaur were among the most abundant fossils found on the American Museum of Natural History expeditions to Mongolia that Roy Chapman Andrews led between 1922 and 1925. The abundance of these fossils has led scientists to believe that Protoceratops was a highly social animal that lived in herds.

The American expeditions also discoveered eggs and nests belonging to Protoceratops. These were the first dinosaur nests ever found, and the discovery was widely publicized. Another famous find was that of a Protoceratops skeleton interlocked with that of a Velociraptor. Whether or not this find represents an actual act of predation, Velociraptor, along with the larger theropods such as Tarbosaurus, would almost certainly have been among the main predators on a small plant-eater such as Protoceratops.

Thanks to the large number of complete skulls of Protoceratops that have been found, scientists have been able to distinguish differences between males and females. In adult males, the frill was more erect and there was a more prominent bump on the snout. This suggests that males used the larger frill, as well as the more protruberant snout bump, as a device to attract females. The bump may also have been used in fights between rival males.

Protoceratops seems to have moved about on all fours. The size and weight of its head and jaws would probably have made a bipedal stance impossible. It would have fed mainly on low growing plant matter, which it broke off with its beak and then chewed with the many teeth in the back of its mouth.